Tactile hallucination is the false perception of tactile sensory input that creates a hallucinatory sensation of physical contact with an imaginary object. It is caused by the faulty integration of the tactile sensory neural signals generated in the spinal cord and the thalamus and sent to the primary somatosensory cortex (SI) and secondary somatosensory cortex (SII). Tactile hallucinations are recurrent symptoms of neurological diseases such as schizophrenia, Parkinson's disease, Ekbom's syndrome and delirium tremens. Patients who experience phantom limb pains also experience a type of tactile hallucination. Tactile hallucinations are also caused by drugs such as cocaine and alcohol.
History and background During ancient Greek times, touch was considered to be an unrefined perceptual system because it differed from the other senses on the basis of the distance and timing of perception of the stimulus. Unlike vision and audition, one perceives touch simultaneously with the medium and the stimulus is always proximal and not distal. By the 17th century, the British empiricist John Locke attributed the word "feeling" with two types of sensation. Weber distinctly identified these two types of sensation as the sense of touch and common bodily sensibility. This distinction further helped 19th century psychiatrists to distinguish between tactile hallucinations and cenesthopathy. During the 19th century, tactile hallucinations were classified as symptoms associated with insanity, organic and toxic syndromes and delusional parasitosis yet there was no identification on how such hallucinations were caused. Neuroscientists such as Dr. Oliver Sacks and Dr. V.S. Ramachandran have analyzed and attributed tactile hallucinations as a dysfunctional perception of the brain as opposed to just a symptom related to insanity. They have contributed significantly to propose tactile hallucinations as the false perception of tactile sensory input creating a sensation of touch with an imaginary object.
In schizophrenia Hallucinations of pain and touch are very rare in schizophrenic disorders but 20% of patients with schizophrenia experience some sort of tactile hallucinations along with visual and auditory hallucinations. The most common tactile hallucination in patients with schizophrenia is a sensation in which a patch of their skin is stretched elastically across their head. They vary in intensity, range and speed at which they feel this stretching painful sensation. They are usually triggered by emotional cues such as guilt, anger, fear and depression. Other types of tactile hallucinations takes on a sexual form in patients with schizophrenia. Patients with schizophrenia may sometimes experience the feeling of being kissed or the feeling of someone lying by their side in response to their emotion of being lonely. Additionally, they occasionally hallucinate the feeling of small animals such as snakes crawling over their body. Such vivid tactile sensation of an object that is not present results from the unsuccessful attempt of the brain trying to perceive objects that are novel and that represent unreal situations usually triggered by guilt and fear. Patients with schizophrenia also have a hard time portraying emotions as they divert most of their energy to control the pain from their tactile hallucinations. Olfactory and tactile hallucinations correlate with one another in terms of prevalence in patients with schizophrenia. One particular study was conducted by Langdon et al., in which the prevalence of olfactory hallucinations and tactile hallucinations was analyzed in two distinct clinical samples of patients with schizophrenia. One of the samples contained patients with schizophrenia with tactile hallucinations as reported by the World Health Organization while the other sample contained cases with negative and positive types of tactile hallucinations in patients with schizophrenia. It was concluded that about 13% to 17% of patients with schizophrenia experience olfactory and tactile hallucinations. The study reported that socio-cultural factors influenced self-reporting of tactile hallucinations. Since hallucinations in general were feared as a symptoms of insanity, patients were reluctant to seek help for such symptoms. Moreover, the study concluded that tactile hallucinations were usually accompanied by several other hallucinations associated with different modalities such as taste and sight. However, the study failed to recognize the pathophysiology of tactile hallucinations in individuals with schizophrenia.
In Parkinson's disease
About 7% of individuals with Parkinson's disease (PD) also experience mild or severe types of tactile hallucinations. Most of these hallucinations are based on the sensation of a particular kind of animal. Several case studies were conducted by Fénelon and his colleagues on patients with PD that had tactile hallucinations. One of his patients described that he sensed "spiders and cockroaches chewing on his lower limb" which was rather painful. Several other patients felt that there was a parasitic infestation of their skin which caused lesions on their skins due to the obsessive need of itching. Fénelon also analyzed the particular types of tactile hallucinations experienced, the timing of such experience and certain drugs that could eliminate such experience. It was concluded that patients with both PD and tactile hallucinations not only experienced sensations elicited by insects under their skin but also by vivid tactile sensations of people. These hallucinations were aggravated during evening times due to altered arousal states and were alleviated by dopaminergic treatment such as the intake of clozapine. The study also explains that the pathophysiology of tactile hallucinations is uncertain, however, such hallucinations can be attributed to narcoleptic rapid eye movement sleep disorders due to its concordance with visual hallucinations. Moreover, it emphasizes that individuals who have had PD for a longer period of time have a more severe form of tactile hallucinations than with individuals who have succumbed to this disease for just a short period of time. Clinical drugs used as an antiparkinsonian agent such as Trihexyphenidyl are known to create tactile hallucinations in patients with PD.
Restless legs syndrome
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